i know there are many theories on 358/2 days and i dont know if this has been done but, maybe the /2 means half, so it means 358 and a half days, and that is how long Roxas was in the organisation i believe wasnt he. So maybe it has nothing to do with 14th member, because we would of heard of her in khCoM and kh2, or maybe it was the 14th member that told him about Sora and where he is and that he is alive and that is why he left.